As well known, a seat belt device restrains an occupant to a vehicle seat with a webbing unwound from a retractor in order to protect the occupant when the vehicle comes into collision. In this seat belt device, a locking mechanism is actuated to stop the rotation of a reel when deceleration in the horizontal direction exceeding a predetermined value is exerted on the vehicle. Often employed as an acceleration sensor for detecting such deceleration is a weight type acceleration sensor having a sensor weight which tilts to the rolling-over side due to the deceleration in the horizontal direction. There are also various types of electrical or electronic acceleration sensors.
A seat belt retractor may be incorporated in a seat back. As a result of incorporating the seat belt retractor in a seat back, a webbing can fit well the occupant, regardless of the inclined angle of the seat back.
In such a case that the seat belt retractor is incorporated in the seat back, a weight type acceleration sensor cannot be employed. That is, when the seat back is inclined, the sensor weight is also inclined, so that the locking mechanism may be actuated. Therefore, conventionally employed as an acceleration sensor in such a retractor incorporated in a seat back is only an electrical or electronic acceleration sensor. This requires electrical wiring to the seat and thus takes a lot of time for assembling the vehicle and for maintenance.